Heretofore in making door and window frame trim assemblies with mitre joints, there has long existed the difficulty of fixedly securiing the mitre joint against displacement or separation of the angularly related trim elements which define such joint. Various efforts have been made to provide a suitable means of anchoring and securing these joints. Anchor blocks have been tried but have been found impractical because they block the continuity of the continuous channelling defined in the trim elements which make up the respective mitre joints of the frame assembly. Screws and fasteners have been tried but have been found ineffective and do not hold properly or function well due to thinness of the metal strips which make up the channel elements at the mitre joint forming a part of the window frame trim assembly. Magnetic spanners have been tried for the mitre joints and have been found costly and ineffective in providing a positive interlock between the adjacent trim elements at the mitre joint.